The present invention relates to a method for fabricating an optical integrated circuit which is constituted by optical devices and an optical waveguide within one chip. In more particular, it relates to a method for fabricating an optical integrated circuit capable of obtaining a current confinement and an opto-coupling efficiency of maximum by using a simple process, in case where an active device such as an optical waveguide and an optical amplifier, or the like.
In a case where a laser diode, an optical detector, and an optical filter, etc., have been integrated within one chip, an optical waveguide has been used as a medium for connecting between them. At this point, the satisfaction of the specifications of the respective devices and the maximization of the opto-coupling efficiency between optical devices and a waveguide are taken into consideration.
When several devices are integrated within one chip, the crystal growth process should be carried out several times, because the specifications required for the different devices are different.
FIGS. 1A, 1B and 1C show methods for fabricating several optical devices within one chip, which are the most frequently used in the prior art. Referring to FIG. 1A, a buffer layer 2, an active layer 3 and a clad layer 4 constituting an optical amplifier is grown over a substrate 1 by use of a primary epitaxy-grown process. A dielectric layer 5 such as SiN.sub.x or SiO.sub.2 is deposited over the clad layer 4 and then is subjected to a photo-etching process to expose the clad layer at which a core layer is formed. The exposed clad layer 4 and the active layer 3 underlying the clad layer 4 is sequentially wet-etched using the dielectric layer 5 as a mask. The core layer 6 constituting an optical waveguide is selectively formed over the portion where the dielectric layer 5 is has been etched.
When growing the core layer 6, the ingredients of the growth layer provided over the dielectric layer 5 are not grown thereover, but are provided from outside the dielectric layer 5 due to a diffusion.
Because the diffusion is not a non-planar growth, a uniform growth layer can be obtained over the whole surface. However, in the case of a selective growth using a dielectric layer, the ingredients provided over the dielectric layer are diffused into the definite surface of a semiconductor layer. Because the relative bonding ratio of ingredients such as In, Ga, As and P is different, the growth condition becomes different in accordance with the distance from the dielectric layer. Therefore, a uniform composition over the surface of the semiconductor layer in the neighbor of the dielectric layer can not be obtained, so that an epitaxy layer of good quality can not be obtained. And, the shape of the growth pattern is difficult to form the opto-coupling characteristic.
Another integrated method is shown in FIG. 1B. Referring to FIG. 1B, a buffer layer 2, an active layer 3 and a clad layer 4 constituting an optical amplifier are sequentially grown over a substrate 1, and then are subjected to a photo-etching process to form an optical amplifier pattern. A core layer 6 and a clad layer 7 constituting a waveguide are sequentially grown over the whole surface of the substrate, thereby simultaneously making horizontal opto-coupling and a vertical opto-coupling. Another integrated method can obtain an excellent opto-coupling efficiency, but in the design of an optical amplifier, the thickness and doping of the growth layers are restricted.
FIG. 1C shows still another integrated method. Referring to FIG. 1C, a buffer layer 2, a core layer 6 of a waveguide, a first clad layer 9, an active layer 3 of an optical amplifier and a second clad layer 4 are sequentially grown over a substrate 1 by use of a primary epitaxy growth process and then are subjected to a photo-etching process to etch the active layer 3 except for an optical amplifier pattern. Then a third clad layer 7 constituting a waveguide is grown by use of a secondary epitaxy growth process.
At this time, considering the loss of a waveguide, the third clad layer 7 may be undoped. However, it has compatibility in that the growth layer 7 serving as the clad layer of an optical amplifier should maintain a proper doping level so as to obtain the excellent gain effect due to a p/n junction.